Ethernet Concepts

How does half-duplex operation work?

By the nature of ethernet collision can happen. A technology called CSMA/CD (carrier sense multiple access collision detect) trying to resolve this by backing off for a random period of time when a collision occurs. After that the station can retransmit. The more crowded an Ethernet segment becomes the more collision will happen.

How does full-duplex work?

If we implement switches in an ethernet network the collision domains are broken. All switch port is a separate collision domain. This way the stations can transmit and receive simultaneously without the possibility of collision (so prevention is not needed either).

What are the specifications of FastEthernet?

maximum speed: 100Mbps

full- / half-duplex

UTP / fiber

UTP max length ~100 meter

Fast EtherChannel (FEC) up to 8 links

What are the specifications of GigabitEthernet?

maximum speed: 1Gbps

full- / half-duplex

UTP / fiber

support legacy ethernet speeds 10/100/1000

Gigabit EtherChannel (GEC) up to 8 links

What are the specifications of Ten-GigabitEthernet?

maximum speed: 10Gbps

only full duplex

Physical Media Dependent (PMD) interfaces:

LAN PHY: Interconnects switches in a campus network, predominantly in the core layer

40Gbps and 100Gbps Ethernet. Both are bonds of multiple Ten-GigabitEthernet.

How does speed and duplex negotiation work?

speed is determined by electrical signaling so that either end of a link can determine what speed the other end is trying to use

Duplex is negotiated: If you want to use autonegotiation both side must be configured to autonegotiate, or else one side will NEVER get information about the other end’s duplexity.
If autonegotiation fails a switch port always falls back to half-duplex because it offers collision detection.Speed and duplex mode can be configured or negotiated only on switch ports that support twisted-pair cabling.

What is “switchport auto negotiation” command used for?

The same as duplex and speed coomand. When autonegotiation is enabled, the port automatically detects the speed or pause method, and duplex of incoming signals based on the link partner. You can also detect link up conditions using the autonegotiation feature.

What is the numbering convention of a Cisco interface (ie.: Fa1/0/14) ?

interface type member/module/number

In the example: the 14th FastEthernet port on the first switch in the stack

What are the 2 ways to configure multiple interfaces at the same time?

How can you check which type of ERROR-CONDITIONs are going to be recovered automatically?

show errdisable recovery

What is the point of reenabling an errdisable port automatically?

When it is more important to keep a link up until the problem can be resolved.

How can you check port status effectively?

show interface

show interface status

What does the protocol and status means in a show int desc output?

protocol: physical or datalink layer

status: Layer 2

How can you notice a duplex mismatch?

“Runts” and “Input errors” are visible in the show interface output.

What are the characteristics of Cisco Discovery Protocol?

it only work in one direction; advertisements are sent out periodically toward any listening device but nothing expected in returnSo if you want to fill your CDP table faster, you need to configure the timer of the neighbor devices.

works in Layer 2

by default CDP advertisements are sent out every 60 seconds

advertisements sent to a multicast address: 01:00:0C:CC:CC:CC

CDP is enabled by default

How do you turn off/on CDP?

It can be configured globally or under a specific interface

What is Layer Link Discovery Protocol (LLDP)?

Same as CDP but based on IEEE; work in multivendor network.

What are the characteristics of LLDP?

disabled by default (on Catalyst switches)
You can check with: show lldp

How do you turn on/off LLDP?

globally: lldp run

globally: no lldp run

PoE (Power over Ethernet)

What type of PoE methods exist?

ILP (Cisco Inline Power) – 7W

PoE (IEEE 802.2af) – 15.4W

PoE+ (IEEE 802.3at) – 25.5W

UPoE (Cisco Universal PoE) – 60W

How can a PoE device detect a peer?

When a switch port is down, there power is also disabled ALTHOUGH the switch tries to detect whther a powered device is connected.
It provides small voltage and measures the resistance wether the currect is being drawn by any other device.

How much power is sent?

By default the switch offer 15.4W / port. The peer can ask for more using CDP or LLDP advertisements and requests (it can ask up to 30W – 802.3at).

How do you configure PoE?

power inline auto/static max MILIWATTS

auto: the peer asks for power (CDP / LLDP)

static: you configure the exact power amount

max: you can configure the maximal power that could be asked by the peer (ie: 4000 = 4W)